1. Field of the Invention
The present invention relates to a method of controlling the valve operating characteristic and the air-fuel ratio to moderate a torque shock produced upon switching of the air-fuel ratio in an internal combustion engine in which the valve operating characteristic for intake valves can be switched between a plurality of operating condition-suited valve operating characteristics suited to an engine operating condition represented by at least one of an engine revolution number and an engine load, and the air-fuel ratio can be switched between a relatively lean level and a relatively rich level.
2. Description of Prior Art
It is conventionally known that if the air-fuel ratio is switched between the relatively lean level and the relatively rich level, a torque shock is produced. In order to moderate such a torque shock, it is a conventional common practice to gradually change the air-fuel ratio, or to retard the igniting time point in promptly switching the air-fuel ratio. It is also already known that in changing the air-fuel ratio from a rich air-fuel mixture state to a lean air-fuel mixture state, the amount of air bypassing a throttle valve is increased without changing of the amount of fuel supplied, thereby moderating the torque shock, as disclosed in Japanese Patent Publication No. 488/1991.
In the technique in which the air-fuel ratio is gradually changed as in the prior art, an increase in amount of NOx in an exhaust gas in the middle of switching between the lean state and a stoichiometric state cannot be avoided. Further, in the technique disclosed in Japanese Patent Publication No. 488/1991, it is conventionally difficult to insure the bypassing air amount by provision of only a bypass passage connected to an intake passage, and it is necessary to newly provide a bypass passage having a bypassing air amount control valve therein. Moreover, if the bypassing air amount is increased during a low-load operation using a lean air-fuel mixture, it is difficult to control the throttle opening degree and hence, it is difficult to maintain the stability during an operation with a small intake air amount, such as during an idle operation and during a partial operation.